1. Field of the Invention
The present invention relates to an ink jet recording apparatus.
2. Related Background Art
Conventionally, many methods of supplying ink to a recording head which are used in ink jet recording apparatus are suggested and are in practical use.
One of the ink supplying methods is a method of supplying ink from an ink tank via a tube to a recording head. The ink jet recording apparatus are equipped with a recording head on their carriages which reciprocate, and mainly adopt a so-called serial scanning system for carrying out recording while the recording head is being reciprocated. For this reason, in the method of supplying ink via the tube, flow of the ink in the tube is influenced by the moving of the carriage, discharge of the ink from the recording head becomes unstable. For this reason, according to heightening of a recording speed, a unit for suppressing oscillation of the ink in the tube is necessary.
Further, the tube whose length is not less than a moving length of the carriage is necessary, and thus a space where the tube can move is secured in the device. This causes enlargement of the device. Air is occasionally mixed into the tube as long time passes, and thus in order to avoid a failure due to this, a tube material tends to have narrow selectivity, and become expensive. When a recovery operation is performed in order to exhaust the air in the tube, a large quantity of the ink which is not used for recording is moved from an ink supply source such as an ink tank. This causes an increase in useless ink and thus deteriorates cost-effectiveness.
In recent years, such a tube is eliminated, and a so-called on-carriage system (hereinafter, it is also described as a head tank on-carriage system) is invented, and it is in practical used in various forms. The head tank on-carriage system uses a cartridge 102 which is provided detachably to a carriage 101 in a form that a recording head is integral with an ink tank (the recording head and the ink tank are integral or can be separated from each other) as shown in FIG. 7.
The on-carriage system has an extremely simple constitution, and since an ink supply passage is included in the cartridge 102, the ink jet recording apparatus can be miniaturized and its cost can be reduced. Moreover, the ink supply passage can be designed to be short, and a number of portions where a moving direction of the carriage and a supply direction of the ink match with each other is extremely small. For this reason, unstable discharge due to oscillation of the ink at the time of high-speed recording can be greatly improved.
When the ink jet recording apparatus are, however, tried to be loaded with a large quantity of ink, a capacity of the ink tank necessarily becomes large, and the increase in the capacity of the ink tank increases a weight of the carriage in the head tank on-carriage system. When the weight of the carriage increases, a driving load of the carriage increases, and thus a driving source such as a carriage driving motor is enlarged. As a result, the ink jet recording apparatus is also enlarged. Moreover, the enlargement of the driving source causes an increase in power consumption.
On the other hand, in small-size ink jet recording apparatus, a size of a carriage becomes small, and thus a quantity of ink to be loaded into the carriage becomes extremely small. As a result, such a defect that a user should frequently replace an ink tank with a new one is estimated. When the ink tank is frequently replaced with new one, a lot of used ink tanks should be discarded. This becomes a problem to be solved because this does not go with the current of the time that environmental conservation is pursued.
A constitution shown in FIG. 8, namely, an intermittent ink supply system (hereinafter, it is simply described as a pit-in system) is, therefore, devised as a unit which solves the problem. In this system, a carriage 112 which is guided to a guide shaft 111 of a printer is equipped with a recording head 113 and a sub tank 114, a main tank 115 is provided separately from the sub tank 114, and the main tank 115 and the sub tank 114 are connected in a predetermined position if necessary so that the sub tank 114 is replenished with necessary ink from the main tank 115.
According to the pit-in system, since the weight of the carriage 112 can be lightened, the carriage 112 can be moved at high speed. Further, since the main tank 115 which houses ink to be supplied to the sub tank 114 is provided, a quantity of the ink to be loaded into the ink jet recording apparatus can be increased to the extent permitted by a space without influencing the weight of the carriage 112. Moreover, it is not necessary to connect the sub tank 114 and the main tank 115 using a tube, the constitution of the device becomes very simple.
The most important technical point in the pit-in system is to supply ink to the sub tank without a problem. That is to say, the most important element technique is how to control the supply of ink until the sub tank 114 is full when the sub tank 114 is connected with the main tank 115.
As one example of this technique is a method of arranging a sensor, for detecting a quantity of the ink in the sub tank, into the sub tank, and detecting a quantity of the ink to be suppliable to the sub tank at the time of pit-in so as to control a supply system of the ink. The mechanism for this method, however, becomes very complicated, delicate and expensive.
Another example is a method disclosed in Japanese Patent Application Laid-Open Nos. 2000-334979 and 2000-334982. In this method, when intermittent supply is carried out, the whole ink in the sub tank is once drafted, and a certain constant quantity of the ink corresponding to the capacity of the sub tank is supplied to the sub tank. According to this method, it is not necessary to add a device or a mechanism for detecting an ink quantity in the sub tank, but a total quantity of the discarded ink exhausted every time of the intermittent supply becomes huge. For this reason, it is necessary to enlarge a reservoir portion for the discarded ink, and thus when an ink jet recording apparatus is desired to be miniaturized, constraint on design becomes large.
As a technique which solves such a problem, therefore, it is suggested that a vapor-liquid separating film is arranged in the sub tank. The ink jet recording apparatus adopting the pit-in system using the vapor-liquid separating film is explained below with reference to FIGS. 9A and 9B.
As shown in FIGS. 9A and 9B, a sub tank unit 120 has a recording head 121 having a plurality of nozzles for discharging ink based on a control signal, and a sub tank 122 for holding the ink to be supplied to the recording head 121. The sub tank unit 120 is provided with a hollow ink supply needle 126 so that its root is positioned in the sub tank 122, and with a suction passage 124 for connecting an inside of the sub tank 122 with an outside so that the suction passage 124 is connected with the sub tank 122. A boundary portion between the suction passage 124 and the sub tank 122 is provided with a vapor-liquid separating film 125 having such a property that it does not allow liquid to pass but allows vapor to pass. Further, the sub tank 122 houses an ink absorber 123, and the ink is held in a state that it is absorbed by the ink absorber 123 in the sub tank 122. The sub tank unit 120 may be provided integrally with a carriage, not shown, or may be provided detachably from the carriage.
As the ink absorber 123, a polypropylene fiber material which is disclosed in Japanese Patent Application Laid-Open No. 2000-296624 is used because it is chemically stable and its cost is inexpensive. Moreover, as disclosed in Japanese Patent Application Laid-Open Nos. H6-255121 and H6-255121, density of the ink absorber 123 composed of the fiber material is set to 0.06 to 0.15 g/cm3 which is suitable for holding the ink.
On the other-hand, an ink supply recovery unit 130 is fixed to an arbitrary position of the ink jet recording apparatus so as to be separated from the sub tank unit 120. The ink supply recovery unit 130 includes a main tank 131 for housing ink to be supplied to the sub tank 122, and a pump 132 for forcibly sucking air and/or ink from the inside of the sub tank unit 120. The pump 132 is connected with a discarded ink reservoir 133 for housing the ink sucked from the sub tank unit 120.
In a home position of the carriage, a cap 134 is arranged in a position opposed to the recording head 121, and a suction joint 135 is arranged in a position opposed to a suction socket 124a of the suction passage 124. Further, a supply joint 136 is arranged in a position opposed to the ink supply needle 126.
The cap 134 caps the recording head 121 at the time of an recovery operation of the recording head 121 and a non-operation of the ink jet recording apparatus, and is it is connected with the pump 132. The suction joint 135 is connected with the suction socket 124a at the time of the ink supply to the sub tank 122, and it is connected with the pump 132. The supply joint 136 is connected with the ink supply needle 126 at the time of the ink supply to the sub tank 122, and it is connected with the main tank 131. The suction joint 135 and the supply joint 136 can be operated independently from the cap 134.
At the time of a normal recording operation, as shown in FIG. 9A, the sub tank unit 120 and the ink supply recovery unit 130 are separated from each other, and while the carriage is being moved, the recording head 121 discharges the ink so that the recording is carried out.
At the time of non-recording, the carriage moves to the home position, and the cap 134 caps the recording head 121. As a result, the ink from the recording head 121 is prevented from evaporating. Moreover, at the time of the recovery operation of the recording head 121, while the cap 134 caps the recording head 121, the pump 132 is driven, so that the ink is forcibly sucked from the recording head 121. As a result, ink whose viscosity increases and foreign matters are removed from the insides of the nozzles of the recording head 121, and thus the insides of the nozzles are maintained clean.
Further, when the ink is supplied to the sub tank 122, as shown in FIG. 9B, the suction joint 135 and the supply joint 136 are connected with the suction socket 124a and the ink supply needle 126, respectively, and in this state, the pump 132 is driven. As a result, only air is sucked out of the sub tank 122 due to the property of the vapor-liquid separating film 125. Since a negative pressure in the sub tank 122 accordingly become high, the ink in the main tank 131 is sucked into the sub tank 122, and thus the ink is supplied to he sub tank 122.
According to the constitution which sucks the air in the sub tank 122 using the property of the vapor-liquid separating film 125 and supplies the ink into the sub tank 122, when a quantity of suction by the pump 132 is not less than an internal capacity of the sub tank 122, only air is sucked out of the sub tank 122 regardless of a quantity of the ink remaining in the sub tank 122. Further, the ink is supplied from the main tank 131 until the sub tank 122 is filled. Since the sub tank 122 can be filled only by sucking air whose quantity is not less than a certain quantity from the sub tank 122, it is not necessary to control a suction quantity. The sub tank 122 can be, therefore, replenished with the ink easily on principle.
When the constitution using the vapor-liquid separating film is applied to actual ink jet recording apparatus, however, the following problems arise.
In the pit-in system, the ink sucked into the sub tank is temporarily held by the ink absorber. A quantity of the ink to be held is determined by a product concept of the ink jet recording apparatus and a balance of the entire system but is not determined physically. For example, as to a small-size ink jet recording apparatus which is suitable to adopt the ink supply system using the pit-in system, a card-size printer is considered as an example.
The ability of the recording head is supposed such that an arrangement pitch of the nozzles is 1200 dpi and a discharge ink volume per 1 dot is 4.5 pl. When such a recording head is used so as to carry out predetermined printing on a whole surface of a card-size sheet (about 54 mm×85 mm), ink of about 0.07 ml is necessary. When an quantity of ink to be consumed by a process for stabilizing the discharge of ink, such as preliminary discharge and a recovery operation, is added to the ink of about 0.07 ml, the ink of about 0.083 ml is actually necessary for one sheet. That is to say, the ink of 0.083 ml as a precise amount per one sheet should be prepared for the ink absorber.
According to experiments by the inventors and the others, a volume of the ink absorber should be about 1.4 times as large as the volume of the ink to be held. That is to say, in order to enable the ink absorber to hold the ink of 0.083 ml, the volume of the ink absorber may be 0.12 ml.
While the consumption and supply of the ink are repeated, however, air is not replaced by the ink and remains in the sub tank and thus a capacity of the air increases. A quantity of storable ink (a quantity of suppliable ink) gradually reduces, and finally it is found that the ink of a quantity necessary for printing onto one sheet cannot be secured in the sub tank. In order to prevent this, a volume of the ink absorber may be enlarged so as to have sufficient margin, but this does not meet the original object of the pit-in system. Further, such a problem that a supply quantity of the ink gradually reduces is not limited to the case where the vapor-liquid separating film is used, and arises in all the ink jet recording apparatus in which the sub tank houses the ink absorber.